I read this trick/bug 3years ago on ... some like www.lineage2-online.com , it always worked for me. An easier way to explain it is when you dont want your speed lowered and breath gauge appear then do this:
Stand near the edge of the water (Not touching it) and click on the other side (Which direction you want to go to) and click that edge there. If done correctly, client thinks your still on the ground, just with swimming effects.
You can also experiment with this. For example if your not near the water or somewhere high or not seeing the other edge then instead of clicking the edge click higher than the water le vel. Basicly clicking the air or sky, not too high though.